1. Field of the Invention
The present invention relates to an anisotropic electroconductive adhesive that is suited to high-density package employed in applications such as connections between IC chips and circuit boards, and more specifically relates to a multilayer anisotropic electroconductive adhesive capable of improving connection reliability to a level corresponding to a fine pitch, and also to a method for manufacturing this adhesive.
2. Description of the Related Art
A need for reduced weight and thickness has recently arose in connection with electronic equipment and devices containing mounted IC chips because of new developments related to such electronic equipment, and there is also a need for higher density in boards for mounting such IC chips.
Examples of methods for the high-density package of IC chips include flip-flop methods, methods featuring the use of anisotropic electroconductive adhesives, and face-down methods in which IC chips are provided with solder bumps, and such IC bare chips with solder bumps are placed on circuit boards and secured by being passed through a solder reflow furnace or subjected to the action of a heat press.
Of these methods, the simple connection techniques employing anisotropic electroconductive adhesives have come to be used. According to these techniques, an anisotropic electroconductive adhesive is placed between an IC chip and a board, electrical conductivity between electrodes constituting vertical arrays is provided through the intermediary of the electroconductive materials contained in the anisotropic electroconductive adhesive by applying heat and pressure, and electrical insularity is maintained between adjacent electrodes on the left and right by preventing contact between the electroconductive materials.
However, the pitch between the electrodes of IC chips is 10-100 .mu.m, which is less than the pitch between LCD electrodes. In addition, applying heat and pressure to the anisotropic electroconductive adhesive imparts fluidity to the anisotropic electroconductive adhesive disposed between the electrodes constituting vertical arrays, and forces it into the gaps between the adjacent electrodes on the left and right. This is accompanied by the flowing of the electroconductive materials, thus increasing resistance to conduction, making conductivity impossible, or causing short circuits as a result of contact between the electroconductive particles flowing between adjacent electrodes on the left and right.
Anisotropic electroconductive adhesives must also be applicable to IC stud bumps, whose surface area has recently been reduced to 1240 .mu.m.sup.2 (40 .mu.m diameter), which is 1/3 to 1/4 of the area of the previously used COG (Chip On Glass) plating bumps. A resulting drawback is that a large number of electroconductive particles are needed for securing such particles on a bump.
Japanese Patent Application Laid-Open Nos. 61-195179, 1-236588, 4-236588, 6-283225 and the like describe techniques aimed at overcoming this drawback. According to these techniques, an insulating adhesive layer is formed on an anisotropic electroconductive adhesive layer, and the softening point or melt viscosity of the anisotropic electroconductive adhesive layer is raised above that of the insulating adhesive layer, or the fluidity of the anisotropic electroconductive adhesive layer is brought below that of the insulating adhesive layer, making it more difficult for electroconductive materials to flow from between the electrodes constituting vertical arrays.
These techniques, however, are still unable to prevent short circuits from being caused by contact between the electroconductive particles present in the gaps between adjacent electrodes on the left and right. It is possible to reduce the particle diameter of the electroconductive materials, but excessive reduction causes an additional problem of secondary aggregation.
Due to their high contact reliability, thermosetting anisotropic electroconductive adhesives are primarily used at present. Microcapsulated latent curing agents, which contain epoxy resins having functional groups as their essential components and which are obtained by applying polyurethane resins to an imidazole that is inert at normal temperature but is activated under heat, are commonly used with such thermosetting anisotropic electroconductive adhesives.
To manufacture a multilayer anisotropic electroconductive adhesive utilizing such a thermosetting anisotropic electroconductive adhesive, the layer containing a latent curing agent is passed several times through a hot oven. A drawback is that the polyurethane resin coat on the latent curing agent is broken by the heat in the oven, initiating a reaction and making it impossible to obtain a finished product.
To address this problem, it has been proposed to simultaneously apply an anisotropic electroconductive adhesive and an insulating adhesive to a release film through a plurality of nozzles of an extrusion coater or the like. This technique, while effective with a very thin adhesive layer, causes a new problem: a multilayer anisotropic electroconductive adhesive of 25 .mu.m or greater allows only part of the contained solvent to vaporize, resulting in the buildup of residual solvent or in the formation of gas bubbles in the adhesive layer, and adversely affecting the functioning of the finished product.